How to Manage Pests

UC Pest Management Guidelines



Pathogen: Alternaria alternata

(Reviewed 12/13, updated 12/13)

In this Guideline:


Blackmold is characterized by obvious lesions that appear on the surface of ripe fruit. Lesions are light to dark brown and vary from small flecks affecting only epidermal tissue to large, more or less circular, sunken lesions with decay extending into the carpel wall and often into the seed locule. During warm, humid weather the fungus may sporulate to form a black, velvetlike layer on the surface of the sunken lesions.

Comments on the Disease

Blackmold is a disease of ripe tomato fruit that appears in the field after rain or dew. Disease incidence is higher with increased late-season rain; it is most common in late-season processing tomatoes. Fungal spores need 3 to 5 hours of wetness to germinate. After germination they can infect fruit by directly penetrating the epidermis. A crop can be heavily damaged within 4 to 5 days following a period of rain and high humidity. The fungus also readily colonizes any wounds on the fruit, including sunburned areas.


Cultural practices help reduce the damage potential of the blackmold fungus but preventive treatments may be needed for control in areas where it poses a continual problem.

Cultural Control
  • Avoid overhead irrigation late in the season and keep beds dry.
  • Delays in harvest increase the chance of exposure to rain or dew and the incidence of blackmold. Begin harvest as soon as fruit ripens.

Cultural practices that encourage dense leaf canopies and the selection of varieties that develop and retain a heavy canopy, may aid in preventing blackmold by protecting fruit from dew. However, a dense canopy also retains high humidity that favors other fruit molds such as gray mold.

Organically Acceptable Methods

Cultural control is acceptable for use on organically certified produce.

Treatment Decisions

Treatments are most likely necessary in late-harvest fields, in rainy years, or if the fruit is damaged. Apply the first treatment 4 to 6 weeks before anticipated harvest. Two applications may be necessary if harvest is anticipated after mid-September. Check with tomato processor representatives concerning allowed fungicides and rates.

Common name Amount per acre** R.E.I.‡ P.H.I.‡
(example trade name)   (hours) (days)

Calculate impact of pesticide on air quality
Bee precaution pesticide ratings
When choosing a pesticide, consider its usefulness in an IPM program by reviewing the pesticide's properties, efficacy, application timing, and information relating to resistance management, honey bees (PDF), and environmental impact. Not all registered pesticides are listed. Always read the label of the product being used.
  (Bravo Weather Stik, generics) 2–2.75 pt 12 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M5)
  (Quadris F) 5–6 fl oz 4 0
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Apply on a 7- to 14-day interval; make no more than 3 sequential applications before alternating with a fungicide that has a different mode of action. Do not alternate or tank mix with fungicides to which resistance has developed in the pathogen population.
  (Quadris Top) 8 fl oz 12 0
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) + Dimethylation inhibitor (3)
  (Fontelis) See label 12 0
  MODE OF ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitor (7)
  (Dithane M-45) 1.5–2 lb 24 5
  (Dithane F-45 Rainshield) 1.2–1.6 qt 24 5
  (Penncozeb 75DF) 1.5–2 lb 24 5
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)
  (Cabrio EG) 8–12 oz 12 0
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Apply at 7- to 14-day intervals; use higher rate and shorter interval when disease pressure is high.
** See label for dilution rate.
Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment until harvest can take place. In some cases the R.E.I. exceeds the P.H.I. The longer of these two intervals is the minimum time that must elapse before harvest may take place.
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action Group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode of action Group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action Group number.



[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Tomato
UC ANR Publication 3470


R. M. Davis, Plant Pathology, UC Davis
G. Miyao, UC Cooperative Extension, Solano and Yolo counties
K. V. Subbarao, USDA Agricultural Research Station, Salinas, CA
J. J. Stapleton, UC IPM Program and Kearney Agricultural Center, Parlier
B. J. Aegerter, UC Cooperative Extension, San Joaquin County (powdery mildew on field-grown tomatoes)
Acknowledgments for contributions to Diseases:
B. W. Falk, Plant Pathology, UC Davis

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